This invention relates to a semiconductor device. More particularly, it relates to a semiconductor junction capacitance constructed as a part of a bipolar semiconductor integrated circuit (IC) and also to a semiconductor integrated circuit device including the same.
In a linear IC wherein an n.sup.- -type epitaxial semiconductor layer is stacked on a p-type Si (silicon) semiconductor substrate with n.sup.+ -type buried layers disposed in parts between them, island regions are formed so as to be isolated from one another by p-type isolation regions which are formed in a manner to extend from the surface of the n.sup.- -type layer down to the p-type substrate, and p.sup.+ -type bases, n.sup.+ -type emitters, etc., are diffused in the island regions, to thereby form n-p-n transistors. For forming a junction capacitance, either the p.sup.+ -n.sup.+ junction between the base and the emitter of the n-p-n transistor, the p.sup.+ -n junction between the base and the collector, or the p-n junction between the collector and the isolation region has been employed. In the junction capacitance, the reverse breakdown voltage of the junction is inversely proportional to the capacitance per unit area. It is therefore deemed that the use of a junction having a minimum reverse breakdown voltage which can bear a maximum load voltage permitted in the circuit (IC) provides the best area efficiency in the design of the IC.
In the linear IC, usually the breakdown voltage of the base-emitter junction is approximately 6 V and that of the base-collector junction is approximately 40 V. Accordingly, the base-emitter junction or the base-collector junction has been used as may be needed. For devices requiring breakdown voltage intermediate between those of both the junctions, the base-collector junction capacitance has been employed, so that the area efficiency has been very poor.
In view of the above points, the inventors considered that a junction capacitance having a breakdown voltage intermediate between the value (6 V) of the base-emitter junction and the value (40 V) of the base-collector junction would ensure the breakdown voltage of a circuit with an area smaller than in the prior-art devices, which consideration led them to this invention.
Further, the inventors have taken into account the provision of a semiconductor integrated circuit device which is formed without adversely affecting other elements during the formation of the junction capacitance as described above, and also a method of producing the same.